Nuclear pore complexes form immobile networks and have a very low turnoverin live mammalian cells

The nuclear pore complex (NPC) and its relationship to the nuclear envelope (NE) was characterized in living cells using POM121-green fluorescent prot ein (GFP) and GFP-Nup153, and GFP-lamin B1. No independent movement of sing le pore complexes was found within the plane of the NE in interphase. Only large arrays of NPCs moved slowly and synchronously during global changes i n nuclear shape, strongly suggesting mechanical connections which form an N PC network. The nuclear lamina exhibited identical movements. NPC turnover measured by fluorescence recovery after photobleaching of POM121 was less t han once per cell cycle. Nup153 association with NPCs was dynamic and turno ver of this nucleoporin was three orders of magnitude faster. Overexpressio n of both nucleoporins induced the formation of annulate lamellae (AL) in t he endoplasmic reticulum (ER). Turnover of AL pore complexes was much highe r than in the NE (once every 2.5 min). During mitosis, POM121 and Nup153 we re completely dispersed and mobile in the ER (POM121) or cytosol (Nup153) i n metaphase, and rapidly redistributed to an immobilized pool around chroma tin in late anaphase. Assembly and immobilization of both nucleoporins occu rred before detectable recruitment of lamin B1, which is thus unlikely to m ediate initiation of NPC assembly at the end of mitosis.